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The Lake Where You Live: Spiny invader

February 07, 2020 by Ted Rulseh

A number of invasive species threaten our lakes — rusty crayfish, various snails, several aquatic plants. Arguably the most dreaded of them all is the spiny water flea.
These little zooplankton, about a quarter-inch long, can substantially change the food web in a lake. They’re native to Europe and Asia and came to this country in ballast water ejected by ocean-going cargo ships. They were first discovered in Lake Ontario in 1982 and since have spread to some inland lakes here in the north.
We know they’re spread mostly by boats and related equipment not properly decontaminated before launching. They’re predatory, feeding on native plankton like Daphnia that are important food sources for young fish. They’re hard for those young fish to eat because of the sharp, stiff barbs on their long tail. 
As with many invasive species, it takes a while to determine exactly how spiny water fleas affect lake ecosystems. Still, scientists in recent years have discovered a few things about them. The Minnesota Aquatic Invasive Research Center reports that spiny water fleas can reproduce very fast; there can be as many as 100 of them per cubic meter of lake water. 
Because they decimate native plankton creatures that eat algae, their presence can increase the likelihood on nuisance algae blooms.
A new study by the University of Minnesota, published just a short time ago, found that in lakes infested with spiny water fleas and zebra mussels, young walleyes grow more slowly than before. In the study, focused on nine well known walleye lakes, it was found that young-of-the-year walleyes were 12-14% smaller after their first summer than before the lakes became infested. That size deficiency could affect their survival later on, although that has not been proven true.
Studies of fishing gear show that items pulled through the water, like downrigger cables, are more likely to accumulate spiny water fleas than stationary items like anchor ropes. This knowledge can help anglers set priorities when inspecting and cleaning equipment after being on an infested lake.
A Canadian study looking at native zooplankton in lakes from May to December found lower abundance, species richness and species diversity in those creatures where spiny water fleas were present. 
Another Canadian study, at the University of Guelph, found that spiny water fleas can evolve to make their tails more problematic to fish that try to eat them. For example, when warmer water signals greater risk of predation by young fish, the spiny water fleas are born with longer tail spines, larger than those fishes’ mouths.  
There’s still a lot to learn about these invaders and their effects. We do know they are not welcome. The key is that once they infest a lake, there is no known way to get them out. 
All we have is prevention, and that’s a lot harder than, for example, preventing the spread of Eurasian water milfoil by pulling easily visible weeds off a boat trailer. It’s essential to clean, drain and dry the boat and trailer and all equipment that has been in the water of an infested lake.
Ted Rulseh resides on Birch Lake in Harshaw and is an advocate for lake protection and improvement. His Lakeland Times and Northwoods River News columns are the basis for a book, “A Lakeside Companion,” published by The University of Wisconsin Press. Ted may be reached at [email protected].


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